1. Field of the Invention
The present invention generally relates to fluid analysis, and more particularly to a thermal bubble point measurement system and method.
2. Discussion of the Background
Fluid analysis is of utmost importance to the oilfield industry. Production decisions for a new well are largely based on measurements of fluid properties, either performed downhole (e.g., directly on the reservoir fluids) or in the lab (e.g., on a sample acquired downhole). Information regarding the chemical composition, phase diagram (e.g., including information on the amount of dissolved gas), density and viscosity of an oil, is critical to deciding which zones of a particular well are economical to produce, and to planning the right infrastructure for production.
One particular issue of concern is the bubble point (BP) pressure of the oil. At high pressures and temperatures similar to those prevalent downhole, a significant amount of gas (e.g., carbon dioxide and the light hydrocarbons, such as methane, ethane, propane, butane and pentane) can be dissolved in the oil phase. The pressure of the oil typically drops during the production process, which may cause the dissolved gas to segregate into a separate gas phase. This process needs to be performed in a very controlled environment, as hydrocarbon gas is highly flammable and compressible, which can lead to major blow-outs and explosions at a well site. Additionally, the permeability of a gas-oil mixture through a porous rock can be reduced by several orders of magnitude by the presence of bubbles, making production impossible. In order to limit the risks of a blow-out and of permeability reduction due to bubble formation, limitations need to be placed on production rates, and the well must often be pressurized at pressures comparable to or even higher than the BP pressure to limit the amount of gas going out of solution.
It is evidently crucial to understand the phase properties of formation oils, particularly the BP pressure at the prevalent temperature in the well. Currently, such phase analysis is performed in several labs around the world, but usually on samples collected downhole, brought to the surface and often stored for a long time prior to analysis (e.g., as described in N. W. Bostrom, D. D. Griffin, R. L. Kleinberg and K. K. Liang, “Ultrasonic bubble point sensor for petroleum fluids in remote and hostile environments,” Meas. Sci. Technol. 16, p. 2336, 2005). Many techniques exist to detect the bubble point in a laboratory environment. Best current lab practice relies on slow depressurization of the sample, while agitating the fluid with an impeller. Optical detection is typically used for bubble identification. Alternatively, the pressure-volume characteristics of the sample can be monitored to detect the considerable change in compressibility at bubble point (e.g., as described in Bostrom et al. cited above). Preliminary work on implementing phase-separation tests in downhole tools has been performed by Esso (e.g., as described in S. C. Wilmot: “Techniques To Improve The Quality Of Wireline Oil Samples In Wells Drilled With Oil Base Mud”, SPWLA 21st Annual Logging Symposium, June 2000) in order to ascertain oil base mud filtrate contamination of the hydrocarbon sample.
The present invention includes the recognition that sample treatment in phase analysis as is performed in several labs around the world, wherein samples are collected downhole, brought to the surface and often stored for a long time prior to analysis (e.g., as described in Bostrom et al. cited above), is likely to trigger irreversible changes in the composition and phase behavior of the fluid (e.g., asphaltene and wax precipitation), making subsequent measurements of BP pressure less accurate. Accordingly, there is a very strong need for developing a bubble point measurement scheme that could be deployed downhole, making sample acquisition obsolete. Physical size of such a device is important, since integration in a downhole tool imposes stringent limitations on the real estate available. The simplest imaginable measurement to detect the BP pressure includes depressurizing a sample of oil in a controlled manner, while monitoring its content for appearance of bubbles. However, such a measurement can introduce major errors in the determination of bubble point pressure, due to the likely condition of supersaturation. In a supersaturated fluid, a bubble may not form spontaneously despite an ambient pressure lower than the bubble point pressure. To avoid errors due to supersaturation, there is a need for a reliable way of nucleating bubbles. One way of nucleating bubbles is mentioned in the work cited above of Bostrom et al., demonstrating the use of an ultrasonic transducer to both nucleate bubbles by means of cavitation near the bubble point pressure and detect persistent bubbles; however, such method involves significant sample volumes, and because the nucleation occurs in the bulk of the fluid, the exact position of the nucleated bubbles cannot be controlled.
Therefore, there is a need for a method and apparatus (e.g., which also can be referred to herein as a “system”) that addresses the above problems of existing systems, although other problems with existing systems will be apparent from the entire description herein. The above and other needs and problems are addressed by the exemplary embodiments of the present invention.